Biomass Digesters – Qiang He

by Laura Buenning

Researchers get ideas from all sorts of places. For Qiang He, the inspiration for his JDRD project began inside the stomach of a cow. What better place to look for microbes capable of decomposing switchgrass for biofuel production than in the gut of grazing ruminants?

But studies show that mimicking host-microbe relationships in an industrial bioprocessing plant is too complex to be realistic. So, He proposed, rather than looking inside the cow, they study the anaerobic digestion of cow manure as a source of unexplored microbial communities capable of decomposing residual plant material.

He, doctoral students Yan Zhang and Si Chen, and the project’s affiliated LDRD team leaders, David Graham (during year one of the project) and Adam Guss (year two)—both in ORNL’s Biosciences Division—favor an approach called consolidated bioprocessing, or CBP. That approach merges the costly, time-consuming hydrolysis and fermentation methods currently in use. In their scheme, CBP relies on anaerobic digestion to convert complex polymers in feedstocks into smaller organic compounds, including biofuels.

The problem of identifying microorganisms capable of efficient deconstruction of recalcitrant plant biomass remains key to developing efficient cellulosic biofuel processing techniques. Microbial communities found in composting or forested environments have proven unsuitable as they rely on oxygen being present to degrade plant material. Likewise, the leading anaerobic candidate up to this point, Clostridium thermocellum, grows poorly on switchgrass feedstocks.

He comes to this research via his interest in water quality and wastewater treatment. Initially curious about boosting biogas (methane) production from dairy manure, his research group set up six anaerobic digesters and studied what happened as they added increasing amounts of organic/nitrogen-rich poultry waste into the mix. Biogas production increased, as they had hoped, but even more fascinating, the team discovered a predominance of a different type of Archaeamicroorganism from the methanogens usually credited with methane production. Furthermore, the unchanging, robust anaerobic archaeal microbial community stabilized the co-digestion process.

In year two of the project, the team set up six additional digesters using pretreated switchgrass biomass to feed an archaeal microbial community. Preliminary comparison of the archaeal populations in both types of digesters revealed one group thrives better on switchgrass biomass than it does in the control digesters fed only dairy and poultry waste.

If further study reveals these and other microbes are specifically involved in the bioconversion of switchgrass, He’s team will use gene analyses to learn how each organism functions in the community.

“The technology we use has the capacity to identify functional genes important for lignocelluloses decomposition,” He says. “Functional characteristics will be linked to community composition to give us a more complete picture of ‘who is doing what’.”

The organisms and genes identified in the JDRD project will help Guss’ team distinguish the enzymes important for designing organisms with metabolic pathways capable of converting lignin and biomass inhibitors into useful products.